The present invention relates to a method for selecting an attack pose for a working machine having a bucket, such as a wheel loader. In particular the present invention relates to a method which evaluates a set of possible attack poses for assessing a loading condition of relevance for the potential filling rate of the bucket. The invention also relates to a method of loading a bucket by use of said method for selecting an attack pose and a corresponding bucket trajectory.
Automatic handling of heterogeneous piled materials is a core component in many construction and mining applications. A typical work cycle of an autonomous wheel loader working in these applications consists of or comprises three repeated tasks: loading, hauling, and dumping. Hauling between the load and dump points can be handled in a number of ways, whether by GPS-waypoint following, or some more flexible approach from the rich literature on mobile-robot navigation. Dumping is relatively straightforward and can, in principle, be performed with preprogrammed motions. Efficient loading is a harder problem than the dumping sequence, and no practical solution for fully autonomous vehicles exists today. For economical and environmental reasons, it is important that the bucket is filled maximally in each load cycle and that the mechanical stress on the machine is minimized. When an automated wheel loader approaches a gravel pile, then, it should evaluate potential attack poses; i.e., positions and orientations at which it is efficient to approach the pile. Also, when used as an operator support function, the same functionality is important to evaluate any potential attack pose and inform the user about the quality of choice.
An example of a prior art method for determining an excavation strategy is presented in U.S. Pat. No. 6,167,336. According to the method a concavity measure is determined by assessing a quota of the volume inside a bucket divided by the total bucket volume. The measure is determined based on the volume in the bucket when the front corners of the bucket touches the pile. Hence a very limited information of the pile is considered for the determination of this concavity measure. Since filling of the bucket is determined by the shape of the pile along the whole trajectory of the bucket through the pile, it is apparent that the measure cannot be used to separate between poor and good trajectories in a real pile. This approach is further quite sensitive to the point sampling of the pile, especially at places that are only mildly convex. Because only a small part of the bucket model enters the pile when the front corners touch the edge, it can easily happen that a high value is obtained even when the bucket enters the pile perpendicularly.
Another example of a method for assessing the quality of an attack pose is provided in “Planning of scooping position and approach path for loading operation by wheel loader”, Shigeru Sarata, et. al., 22nd International Symposium on Automation and Robotics in Construction ISARC 2005—Sep. 11-14, 2005, Ferrara (Italy).
In this paper the shape of the pile at a trajectory of the bucket through the pile is considered. The method uses an approach where a value related to the torque around the center of the bucket is calculated and used as a parameter for determining the quality of an attack pose. It is apparent that important information is lost by the proposed method, since the load distribution cannot be derived from the studied torque measure.
It is desirable to provide an improved method for selecting an attack pose which enables provision of high fill rates of the bucket.
A method according to an aspect of the present invention includes the steps of: acquiring three dimensional pile data, generating a set of attack poses; and generating a bucket trajectory through the pile for each attack pose. The acquiring of the pile data can be performed by a use of a 3D range sensor.
The set of generated attack poses and corresponding bucket trajectories are possible attacks. Among the set of attack poses and bucket trajectories one will be selected at each loading operation. After a loading operation is performed, new pile data will be retrieved, a new set of attack poses and bucket trajectories will be determined for enabling selection of an actual bucket trajectory and a corresponding attack pose to be executed.
The method further includes the steps of for each attack pose in said set of attack poses, calculating a measure of a convexity of the pile surface for an area of the pile delimited by a bucked width and a vertical projection of the bucket trajectory, and selecting an attack pose based on said measure of convexity.
An attack pose consists of or comprises an angle of attack of the bucket and a position for the attack. The angle of attack is an angle of the bucket with reference to a negative surface normal, which is a normal pointing inwardly into the pile, hence in the same direction as the attack of the bucket into the pile at the position of attack. The position of the attack is considered to be the point of the pile at which the middle of the bucket in a lateral direction makes contact with the pile.
By considering the convexity of the area of the pile for an area at the surface of the pile delimited by the bucked width and vertical projection of the trajectory, a measure which has a substantial impact on the filling rate of the bucket is evaluated. It is apparent that cross-sections of the pile transverse to the bucket trajectory having a convex shape will assist in providing a bucket load having a convex shape. This is different from the known prior art which do not consider the shape of the pile along the bucket trajectory and therefore cannot be used to determine the convexity of the pile in the relevant area.
In an embodiment of the invention the convexity measure is determined by determining sweep volumes of segments of said bucket. A sweep volume is a volume of the pile within a sweep area at the surface of the pile defined by a width of a segment of the bucket and length extension of the bucket trajectory, that is the vertical projection of the bucket trajectory. The bucket trajectory is the trajectory which the bucket is intended to propagate through the pile in the event the bucket trajectory is selected for execution. The method further comprises the step of calculating said measure of convexity based on said sweep volumes of segments of said bucket.
Specifically the segments may include a central segment, a right wing segment and a left wing segment. The measure of convexity may be calculated based on a comparison between the sweep volumes of the left wing segment, the right wing segment and the central segment.
In an embodiment the width of the bucket is separated into three segments of equal width. Here the measure of convexity may be calculated as CC=(Vc/max(Vr, Vl))−1.
Optionally, a side load measure may additionally be determined for each attack pose. The attack pose may be selected in dependence of the measure of convexity and the side load measure. The side load measure may be based on a comparison between the volumes of the left and right wing segments. The side load measure may be calculated as CS=(abs(Vr−Vl))/(Vr+Vl).
The pile data may be retrieved by retrieving 3D point cloud from a sensor and separating said 3D point cloud into ground data and said three dimensional pile data.
Optionally a ground plane may be fitted to the ground data.
Optionally potential attack poses may be selected among scan points in the vicinity of and preferably at the border of the pile, wherein orientations of the attack pose is selected within a range of less than 20° deviation from a border normal.
Optionally, the sweep volumes are calculated from the ground plane to a pile surface.
Optionally, the sweep volumes are calculated from the trajectory to a pile surface.
In an alternative embodiment, a surface may be fitted to three dimensional pile data within an area defined by a width of the bucket and said bucket trajectory, and the measure of a convexity may be calculated from said surface.
Such an alternative embodiment may include the process steps of:                fitting a quadratic polynomial: zr=axr2+bxryr+cyr2+dxr+eyrto said three dimensional pile data within the area defined by a width of the bucket and said bucket trajectory; and        determining said measure of convexity from said factor a.        
The method may optionally include the steps of:                fitting a plane to the three dimensional pile data within an area defined by a width of the bucket and said bucket trajectory;        constructing a rotated principal frame from a ground frame being rotated around the surface normal of said plane;        mapping the three dimensional pile data into the rotated principal frame.        
Optionally, a measure of side load may be determined from said factor d.
The invention also relates to a method for loading a bucket by use of a construction machine having a bucket, a sensor system for detecting three dimensional pile data, and an automated steering system for enabling propagation of the bucket along a bucket trajectory in a pile, said method including selection of an attack pose and a corresponding bucket trajectory by use of a method for selecting an attack pose as described above and control of the construction machine to position the bucket at said selected attack pose and to run said bucket along said selected bucket trajectory in order to optimize the filling of the bucket. This method is suitable for automatic handling of heterogeneous piled materials by an autonomous wheel loader